The development of various processes for the steam-cracking and catalytic cracking of naphthas and gas oils puts on the market a C.sub.4 cut, which, after butadiene extraction, essentially contains a mixture of butenes, isobutene, butane and isobutane which must be upgraded.
The olefins, butenes and isobutene, can be used in the petrochemical synthesis of more elaborate products (alcohols, aldehydes, acids, nitriles, etc.); however the available amounts of these C.sub.4 cuts are, as a rule, too large or, at least, can become too large for this upgrading method alone.
A second upgrading method consists of recycling the C.sub.4 cut to the cracking (steam-cracking) or catalytic cracking unit, after olefin hydrogenation; however the isobutane of the hydrogenated cut, which amounts to more than 50% by weight of said cut, does not lead to sufficiently valuable ethylene yields and also leads to a substantial production of methane usable only as fuel.
A third upgrading method for said C.sub.4 cut consists, after hydrogenation of the cut, of separating n-butane from isobutane by distillation and recycling only n-butane to the steam-cracking unit, isobutane being left apart for other more advantageous uses, such as alkylation for the production of isoparaffinic gasolines. However the yields of ethylene and propylene, in such a steam-cracking, do not exceed 38 and 20% b.w. respectively, and the production of methane is about 25% b.w.
A fourth method to upgrade the C.sub.4 cut consists of alkylating the olefins of the cut with the isobutane contained therein to maximize the gasoline yield. However, in the alkylation step, the behaviour of isobutene is not so favorable as that of the butenes as concerns the production of high octane gasoline; as a matter of fact, the Research (or Clear) octane numbers of the alkylates obtained from normal butenes are higher than those obtained from isobutene.
This fourth method can be improved, in a first time by first subjecting the C.sub.4 cut to polymerization while limiting as much as possible the global conversion of the normal butenes of the cut to less than 10% and converting more than 90% of isobutene (preferably, more than 92%); this hydrocarbon is essentially converted to isobutene dimers and trimers; thereafter, the so-obtained product is fractionated into a first fraction which is fed to the alkylation unit, and, a second fraction which is fed to the gasoline pool after partial or total hydrogenation. The first fraction, which is fed to the alkylation unit, contains, in major part, butane and isobutane as well as the butenes which have not reacted during polymerization. A process of this type is described in the British patent application No. 2,017,746.